1. Field of the Invention
The present invention relates to a process for recovering rare gases using a gas-recovering container and, more particularly, relates to a process of recovering a rare gas in a rare gas-containing exhaust gas discharged from an equipment using said rare gas, and introducing the recovered rare gas-containing exhaust gas into rare gas separation and purifying equipment and therein separating and purifying the rare gas.
The rare gas in the present invention is krypton or xenon, or a mixed gas thereof.
2. Description of the Related Art
In processes for producing semiconductor integrated circuits, liquid crystal panels, solar batteries and panels thereof, and semiconductor products such as magnetic disks and the like, apparatuses capable of generating plasma in an atmosphere of an inert gas and carrying out various treatments of semiconductor products by the plasma have been widely used.
Conventionally, in these treatments, argon gas has been used as an inert gas. Recently, krypton or xenon has been used as an inert gas for carrying out more sophisticated treatments (hereinafter, the term of rare gas in the present specification indicates krypton and xenon). Krypton and xenon are very expensive gases because of their present proportion in the atmosphere and complicated separation processes. In order to economically perform the processes using such valuable gases, it is essential to recover used krypton or xenon at a high recovery rate, separate and purify it, and submit to circulating use thereof. Further, krypton or xenon obtained after recovering, separation and purification thereof is used in a highly-purified state such that it has an impurity concentration of not more than 100 ppm.
Exhaust gas containing a rare gas (krypton and xenon) mainly comprises the rare gas and either nitrogen or argon, wherein the rare gas is a subject for separation and purification. Exhaust gas caused by plasma oxidation further comprises several % of oxygen in addition to the above-described gases. Furthermore, exhaust gas caused by plasma CVD comprises metal hydride type gases. Moreover, exhaust gas sometimes contains slight amounts of impurities and, as a reaction by-product, moisture, carbon monoxide, carbon dioxide, hydrogen, hydrocarbon and the like.
Known examples of a process for separating and recovering an objective gas from a mixed gas may include a cryogenic distillation process, a pressure swing adsorption process (PSA), a membrane separation process and a combination of these processes. For example, in the case of preparing oxygen and nitrogen as a product using air as a raw material by the cryogenic distillation process, pressurized air is cooled to about −190° C. by heat exchange and introduced into a distillation column, and oxygen and nitrogen are separated by conducting fractionating in the distillation column and taken out. The cryogenic distillation process has advantages of easily preparing gases having high purity and preparing large amounts of gases at a low cost.
In the meantime, in the case of preparing oxygen as a product using air as a raw material using the PSA process, air is passed through, under pressure, using zeolite as an adsorbent and thereby nitrogen, which is an easily adsorbing component, is adsorbed and fixed to the adsorbent, so that oxygen, which is a hardly adsorbing component, is taken out from the adsorbent layer. When the nitrogen-adsorbed adsorbent is placed in sufficiently low pressure conditions from the air-passing step, nitrogen is desorbed from the adsorbent and thereby can be in a reusable state. The PSA procedure of repeating the adsorption procedure under relatively high pressure and the regeneration procedure under relatively low pressure has advantages such that the amount of products generated per adsorbent is easily enhanced and the apparatus thereof can be made compact because adsorption and regeneration can be changed for a short time.
The membrane separation process is a process of passing air as a raw material to a polymer resin membrane having high affinity to oxygen, or to the inside of a membrane having a physical pore diameter smaller than nitrogen, and collecting oxygen selectively effused outside the membrane. This process has an advantage such that the apparatus is compact and is not expensive although oxygen having high purity cannot be obtained.
In the process for preparing semiconductor products, before a substrate for treatment is introduced into the inside of a chamber, the chamber inside is made to be in a clean nitrogen atmosphere by, for example, vacuum pumping while passing nitrogen gas through the chamber. Thereafter, the substrate is passed into the treatment chamber. In this event, passing of nitrogen gas and vacuum pumping are continuously carried out for keeping the clean nitrogen atmosphere. Therefore, most of the gas discharged before and during the substrate passage is nitrogen gas.
Thereafter, the passing gas is changed from nitrogen gas to a rare gas (krypton or xenon) and thereby the inside of the treatment chamber becomes a rare gas atmosphere and then treatment is carried out by generating plasma with high frequency electric discharge or the like. That is, when the plasma treatment is carried out, most of the components of the gas discharged from the treatment chamber are rare gases. After high frequency application is stopped to stop plasma and the passing gas is changed into nitrogen, the substrate is taken out. Most of the gas discharged during the time between the plasma stoppage and taking out of the substrate becomes nitrogen gas.
Furthermore, between the treatment chamber and the vacuum pumping system, nitrogen gas is passed through at all times in order to prevent reverse diffusion of impurities generated from the vacuum pumping system. This nitrogen gas is discharged together with the gas discharged from the treatment chamber. Further, in order to prevent air from involving a bearing portion of a vacuum pump, nitrogen gas is passed to the bearing portion and a part of the nitrogen gas passed enters into a vacuum pumping system and is exhausted.
As described above, when the substrate is carried in the treatment chamber and carried out from it, and the treatment chamber is in the stand-by operation, most components of the gas exhausted are, for example, inert gases such as nitrogen, argon and the like, while the components of exhausted gas in the plasma treatment contain nitrogen or argon, and a rare gas. Here, the gas pressure at each exhausting time is atmospheric pressure.
Accordingly, even if plasma treatment is completed in the treatment chamber and then nitrogen gas or argon is passed through, the amount of the rare gas in exhausted gas is not rapidly decreased. That is, the rare gas concentration in the exhaust gas is always varied, and there is a time lag between the atmosphere in the treatment chamber and the exhaust gas components. Further, into the exhaust gas, a gas for preventing the vacuum pump from involving of the atmosphere, a gas for preventing reverse diffusion and a gas for preventing sedimentation will be added so that the rare gas concentration is decreased by one order of magnitude or more as compared with the gas concentration fed into the treatment chamber.
Exhaust gas containing a rare gas, which is a subject for recovering, separating and purifying, mainly comprises a rare gas, and nitrogen or argon, and in plasma oxidation, exhaust gas further contains several % of oxygen in addition to them. Further, in plasma CVD, exhaust gas further contains a metal hydride type gas and in reactive ion etching, exhaust gas further contains a halogenated hydrocarbon type gas. Furthermore, exhaust gas may contain slight amounts of impurities and, as a reaction by-product, moisture, carbon monoxide, carbon dioxide, hydrogen, hydrocarbon and the like. Therefore, it is necessary to previously exclude gas components which deteriorate the adsorption properties of activated carbon from the above gas components.
When exhaust gas is recovered in a gas container, as described above, the exhaust gas amount is very large as compared with the amount of the gas introduced into a treatment chamber and thereby the volume of the gas-recovering container is huge. For example, if exhaust gas is assumed to be recovered into a 47 L container with a pressure of 0.1 MPa, even if the gas for preventing sedimentation is not added, the exhaust gas amount corresponding to only about 100 substrates treated can be recovered. In this recovering, the rare gas amount recovered in the recovery container is about 100 L.
On this account, the recovery container needs to be changed frequently and consequently, the increase of the distribution cost of the recovery container will be caused. Further, there is a problem such that the control of a rare gas separation and purifying apparatus will be difficult because the rare gas concentration varies and the rare gas concentration is different in each of recovery containers. Therefore, it is necessary to introduce exhaust gas into the separation and purifying apparatus and to operate the separation and purifying apparatus while measuring a rare gas in a low concentration contained in each recovery container, or it is necessary to separate and purify a rare gas using a purifying apparatus having sufficient redundancy. Consequently, the cost for measuring is added or the size of the separation and purifying apparatus is enlarged.
The plasma treatment time varies depending on semiconductor products. For example, in the case of plasma oxidation treatment by adding several % of oxygen gas to a rare gas, the treating time is determined between from 1 minute to 5 minutes according to the thickness of an oxidation film to be formed. Additionally, the oxidation film thickness varies each several substrates or several ten substrates with the result that the maximum rare gas concentration in the exhaust gas and the time vastly varies each several ten minutes to several ten hours.
As a process for recovering rare gas-containing gas exhausted from rare gas use equipment for producing semiconductor products and separating and purifying the rare gas, proposed are a process for efficiently separating and purifying a rare gas by separating the rare gas and impurities by a process using at least two separation means including membrane separation and adsorption separation and an apparatus used in the process (for example, disclosed in Japanese Unexamined Patent Publication No. 2002-97007).
With regard to the rare gas recovery and purifying apparatus, the above patent literature proposes a device for separating and purifying an objective rare gas from an exhaust gas in which flow rate and rare gas concentration successively vary. Particularly, in the case where flow rate variation or concentration variation, which is not assumed in designing and manufacturing, occurs, the above rare gas recovery and purifying apparatus has a possibility of unstable operation conditions.
Furthermore, because the rare gas recovery and purifying apparatus is designed in the use conditions of an equipment using a rare gas, when the equipment using the rare gas is changed, it is difficult to employ the apparatus as it is, and the apparatus has a problem of lacking in redundancy.
Under the circumstances, it is an object of the invention to provide a process for recovering a rare gas using a gas-recovering container capable of stably recovering rare gas-containing exhaust gas even if the flow rate of exhaust gas or the rare gas concentration in exhaust gas varies, in recovering a rare gas from exhaust gas containing a rare gas, which is a high value gas such as krypton or xenon and used as an atmosphere gas for apparatuses of producing semiconductor products, and then separating and purifying the rare gas.